1. Technical Field
Aspects of this document relate generally to cementitious products, and more particularly for a matter or composition to control or retard the setting of Portland cement or other hydraulic cement or other materials to make cementitious products.
2. Background
The pouring of cement is a delicate balance between creating an appropriate (i.e. smooth or appropriately textured) top surface for the cement and the dry time for the cement. Generally, it is desirable to extend the dry time for cement without reducing the cement's ability to harden with sufficient strength. Cementitious materials, such as is used for building foundations, roads and sidewalks, are different from mortars, adhesives and binders, such as are used to hold bricks together or tile to a surface, generally as a result of the amount of cementitious material included within the mixture. Because mortars, adhesives and binders are generally applied to stick two things together, the set times for mortars and adhesives are generally much longer than for concrete, gunite, plaster, and the like. The compressive strengths required by mortars, adhesives and binders are generally much lower, and the flexural strengths required are generally much higher, than for concrete, gunite, and plaster. To achieve these requirements, the cementitious materials content of mortars and adhesives are typically much less, and the polymers content are much greater, than for concrete, gunite, and plaster.
Adhesives, binders, and mortars are not within the scope of cement as used herein. These type materials would include thinsets, EIFS (exterior foam insulation systems), or the like, which are polymer-dominated. Such materials are not thought of as hydraulic cement dominated materials, such as; concrete, gunite, or plaster. Rather, these are considered ‘adhesive’ and ‘flexural’ materials. Neither can be used in place of the other, as their functions are mutually exclusive. Moreover, adhesives, binders, mortars, and the like, use less hydraulic cement and more polymers, to the point that low amounts of hydraulic cement present in the mix design, behave more as a ‘filler’ material, and moreover, the adhesive or binding ability of these materials are essentially derived from the over-riding polymer. Whereas, the hydraulic cement of concrete, gunite, plaster, and the like, is the predominant main binder of the end product. Mortars, binders and cementitious material adhesives, as the terms are used herein, typically have a range, by weight, of 8%-24% of cementitious materials whereas cements, as used herein, such as concrete, gunite and plaster, have a range, by weight, of 25%-50% of cementitious materials. As a result, the specific compositions and mixtures for set retardants for concrete, gunite, and plaster products are inherently different and react differently from set retardants used for adhesives, binders and mortars.
Set retardants are sometimes used to extend the time for placement, working, and finishing of cement in certain situations, such as moderate to high-temperature environments, or work of large scope. Set retardants may be used to retard setting time, extend setting time, or delay hydration of the cementitious material. The amount and particular type of set retardant used will vary by the specific cement mixture and quantities being used. Changes in the composition of the mixture of the cement can have greater and lesser affects on the ability of any particular set retardant to work efficiently and permit adequate additional time for working the cement to an appropriate finish. Additional components added to a particular cement mixture can significantly change the ability of particular set retardants to operate as intended.
The Ancient Egyptians and Romans practiced the art of making and using of pozzolan, as a cementitious product. The benefits of using pozzolans can be dated back to a time preceding the Roman Empire. Pozzolans are generally high in siliceous or in siliceous and aluminous materials. Pozzolans were used originally with limestone-type materials, such as slaked lime, together with water to make a type of pozzolan cement. Pozzolans include a wide variety of materials. Some pozzolans are naturally occurring such as, but not limited to certain clays, volcanic ashes, and opaline. Natural pozzolan cements were widely used in the western world and elsewhere until at least the early 1940's when it became evident that the advancing technologies in the manufacture of portland cement offered many advantages over pozzolan cement. A significant drawback to pozzolan cement is that it had relatively slow strength gain and slow setting time. This significantly slowed the pace of construction and inhibited its use in many structural applications. Brick or steel were therefore used for most large structures.
Advancements in the manufacture of portland cement compositions eventually reached a level that allowed the user to achieve increased strength gain and more rapid setting times to become one of the dominant construction materials. Further technological advances in the 1960's to 1980's allowed portland cements to be ground finer and the calcining process to become more refined, which beneficially increased the early strength characteristics of portland cement. More recently, however, pozzolans have been used only as supplementary additives or admixtures to be combined with portland cement and not as the primary binder as in the earlier pozzolan cements.
Certain man-made pozzolans have also been used in combination with portland cement to produce cementitious products. Silica fume, also known as fumed silica or microsilica, fly ash, also known as F ash, C ash or ultra-fine fly ash, or cenospheres, are known pozzolans. The chemical and physical mechanisms by which pozzolans react with cement are understood to be a reaction between the pozzolan and the calcium hydroxide compound of the portland cement, which is compound released during the hydration of the portland cement. Pozzolans are known to have little to no cementing properties of their own. However, when pozzolan reacts with free lime or calcium hydroxide in cement, it forms a calcium silicate, calcium alumina silicate, or such similar compound that is less soluble, less reactive, and therefore less susceptible to most given service environment that the cementitious material might be placed.
Polymers have also been known to be mixed with cementitious materials. In mortars and adhesives, for example, polymers have been known for addition as a primary ingredient as a binder to meet the required characteristic properties of mortars and adhesives.